The present invention pertains to organic thin-film electroluminescent devices and a method of manufacture of such devices.
Fabrication of organic electroluminescent devices (OELDs) can be traced back to 1960""s. Representatives of early OELDs are U.S. Pat. No. 3,172,862 to Gurnee et al, issued Mar. 9, 1965; U.S. Pat. No. 3,173,050 to Gurnee, issued Mar. 9, 1965; and U.S. Pat. No. 3,710,167 to Dresner, issued Jan. 9, 1973. These devices comprised a single organic emitting layer having thicknesses above 1 mm and two electrodes (cathode and anode). Alkali metals were used as the cathode materials as they had the lowest work function values.
U.S. Pat. No. 4,356,429 to Tang et al disclosed an OELD with two organic layers, in which the organic luminescent medium consisted of two extremely thin layers separating the anode and the cathode. These comprised one layer injecting and transporting holes and the other layer injecting and transporting electrons and also acting as the organic luminescent zone of the device.
Improvements were then achieved by using different cathode materials. U.S. Pat. No. 4,885,211 to Tang et al disclosed an OELD with its cathode comprising a layer of a plurality of metals other than single alkali metals, at least one of the metals having a work function of less than 4 eV. U.S. Pat. No. 5,608,287 to Hung et al. disclosed OELDs having a conductive electron injector. U.S. Pat. No. 5,668,438 to Shi et al disclosed an OELD in which the materials making up the electron transporting layer (ETL) and the hole transporting layer (HTL) were selected so that the energy barrier for hole injection from the HTL to the ETL was substantially higher than the energy barrier for electron injection from the ETL to the HTL. In that case, light was emitted from the HTL rather than the ETL.
More recently, U.S. Pat. No. 5,776,623 to Hung et al disclosed an OELD containing a transparent electron-injecting electrode. The electrode included a thin non-conductive layer contacting the electroluminescent layer and a conductive transparent overcoat layer. The thickness of the nonconductive layer was selected so that the bilayer acted as an electron injecting contact and provided stability against atmospheric corrosion.
However, only very few patents have dealt with the hole injecting structure. U.S. Pat. No. 5,061,569 to Vanslyke et al disclosed an internal junction OELD in which the hole injecting and transporting zone included a tertiary amine containing at least two tertiary amine moieties and including, attached to a tertiary amine nitrogen atom, an aromatic moiety containing at least two fused aromatic rings. U.S. Pat. No. 5,783,292 to Tokito et al disclosed an OELD in which organic-inorganic composite thin film was used.
It is an object of the present invention to provide an OELD with an increased electroluminescent efficiency and/or improved stability or which will at least provide the public with a useful choice.
This invention comprises an OELD and method of manufacture in which an inorganic buffer layer is included in the hole injecting region.
Preferably, the device comprises in order: an ITO-covered glass substrate which behaves as the hole injector and is transparent and transmissive to optical radiation, a layer of inorganic material as a buffer to the hole injection, an organic single layer or multilayer structure for electroluminescence and carrier confinement, and a layer of low-work-function material as an electron injector which is stable relative to atmospheric corrosion. The buffer layer may also be included in the electron injecting region.